1. Field of the Invention
The present invention relates to electrical and fiber optic cables and methods of manufacturing such cables in long single lengths by providing a multiple sheath with staggered weld joints for increased strength and environmental protection.
2. History of the Prior Art
In the drilling and maintenance of deep oil wells and geothermic wells, it is necessary to periodically lower temperature and pressure probes, gauges and other telemetry equipment into the well to take bottom hole measurements.
Since some of these wells are at depths ranging from 15,000 to 40,000 feet, the tensile strength of the telemetry cable must be extremely high to support the weight of the equipment in addition to the weight of the cable due to the length thereof. Further, the cable may be exposed to extreme temperatures, pressures and highly corrosive materials while inside the well.
The cables that could be used for such deep well logging are of the stainless steel jacketed mineral oxide type cables typically used as heater cables as taught in U.S. Pat. No. 4,137,762, issued Feb. 6, 1979 to William D. Smith.
However, present methods of manufacture of such cable result in cables of limited length. These limitations in length are primarily due to the limited lengths of available strip stock material being used. Further, in order to jacket cable with material such as stainless steel, the strip stock must be rather thin so that it can be bent around the wire and the insulation. This use of relatively thin material will not yield the necessary tensile strength for such deep hole telemetry operations.
Further, if ordinary splicing techniques are used, the tensile strength at the splice joint is less than that of the cable itself and hence can give rise to localized failures.
Also, in the manufacture of such cables shorter lengths naturally occur due to random weld failures along the sheath seam which leaves holes in the seam requiring the cable to be cut at that point. This cutting at relatively short lengths is quite acceptable in the heater cable usage since often there is a use for short cables but would be totally unacceptable in producing telemetry cable for deep hole weld logging operations.
Recently, a second need for long cable lengths with high tensile strength has developed in the form of fiber optic communications cable. Telephone companies are now experimenting with fiber optic lengths in excess of one mile and which are to be buried in trenches thereby subjecting the cable to harsh environments.
Such cables must have considerable tensile strength due to the handling of the long lengths thereof and while high temperatures are not normally present in such usage, other environmental conditions exist such as corrosive earth environments and heavy lateral loads which might tend to crush the cable.
Since it is very difficult to splice fiber optic bundles and since such material exhibits a relatively low tensile strength, it would be necessary to jacket the cables with a protective sheath having high resistance to corrosive surroundings, high tensile strength and resistance to crushing lateral loads.
Still another use for metal jacketed cable is for the protection of power cables in extreme environments where reliability is essential.